Various semiconductor processes require careful control of the amount, i.e., the mass, of material (usually in the form of a gas or vaporized liquid) provided to a work piece during fabrication. As a consequence, devices known as flow sensors have been devised to sense the mass flow of a gas or vapor. Flow sensors can be configured to meter the flow rate of a material, or when combined with control devices control the amount of the material being delivered to a work piece.
The two common types of sensors are pressure-based sensors and thermal-based sensors. Thermal-based sensors are devices which operate on heat transfer principles. A common commercial form incorporates a small diameter tube of capillary-sized dimensions, the tube having two coils of wire wound on the outside of the capillary tube in close proximity to each other. The coils are formed from a material having a resistance which is temperature-sensitive, i.e., has a resistance as a function of temperature. Opposite ends of the capillary tube are in fluid communication with a larger tube which transports the gas or vapor between a source of the gas or vapor and the processing station where the gas or vapor is utilized. A laminar flow element is disposed within the portion of the larger tube called the bypass, between the upstream and downstream connections of the capillary tube to the larger tube. The laminar flow element insures that the flow of gas or vapor through the bypass is laminar flow. As a gas or vapor flows through the sensor predetermined portions of the gas flow through both the bypass and capillary tubes in a predetermined ratio known as a bypass ratio. By sensing the flow rate through the capillary tube, and knowing the bypass ratio, the flow rate through the entire sensor is proportional to the measured flow rate through the capillary tube.
The coils are connected in a bridge-type analog electrical circuit, or to the input of a digital system. The coils can then be heated by an electrical current to provide equal resistances in the absence of flow of the gas, and in the case of an analog electrical bridge-type circuit a balanced condition—e.g., a null output signal. Alternatively, the two coils can be heated by an electrical current, and the two resistances measured with a digital circuit.
Then, with the gas flowing within the tube, within the relevant measuring range of the sensor, the temperature of the upstream coil is decreased by the cooling effect of the gas and the temperature of the downstream coil is increased by the heat first transferred from the upstream coil, and subsequently transferred by the gas or vapor to the downstream coil. This difference in temperature in fact is proportional to the number of molecules of gas per unit time flowing through the sensor. Therefore, based on the known variation of resistances of the coils with temperature, the output signal of the bridge circuit or digital circuit provides a measure of the gas mass flow.
In various circumstances, forms of heat transfer phenomena can introduce substantial error in the measurements of these mass flow meter devices and problems for mass flow controllers. U.S. Pat. No. 3,938,384, issued Feb. 17, 1976 (the “'384 patent”), U.S. Pat. No. 4,056,975, issued Nov. 8, 1977, U.S. Pat. No. 5,191,793 (the '793 patent), issued Mar. 9, 1993, and U.S. Pat. No. 5,279,154 (the “'154 patent”), issued on Jan. 18, 1994 are illustrative of the problem.
As discussed in the '793 patent, at relatively elevated pressure levels of the gas, the error introduced by free convection of the gas within the tube becomes relatively dominant. The result, for such higher pressure levels, is a substantial error due to such convection when the device is tilted with respect to the direction of gravity. As discussed in these patents, at relatively lower pressures, the effects of this sort of convection are not substantial; however, the error introduced by free convection by the ambient gas outside the tube becomes a dominant source of error with variations in the attitude of the device with respect to gravity. In the '384 patent, this sort of convective effect is addressed by encapsulating the capillary tube with the coils thereabout, in the vicinity of the coils, with an open cell foam material. Although, as indicated in the patent, the advantages of that approach are substantial, it does bring certain detriments. First, it slows the response of the device as a result of the presence of the foam material. Second, the calibration of the device can shift with time as the foam changes its chemical composition or its degree of contact with the coils and conduit. Third, it reduces the gain of the device.
A general approach to the convection outside the conduit, of which the just-mentioned approach may be considered a specific form, involves the use of various materials to contact the coils in order to keep convective currents from transferring heat externally from one coil to the other. In addition, one must usually calibrate a sensor in a specific orientation and zero the device upon orientation change. These general approaches typically is unsatisfactory for a variety of reasons, the most important one usually being the reduction of the level of response of the device to changes in flow, and require human or system interaction depending on the type of interface to the device.
Yet more generally, flow meter devices such as those discussed above, are commonly enclosed in some type of container to isolate their sensitive parts from outside air currents and outside localized sources of heating or cooling. This, of course, is a distinct concern from the effects of convection immediately adjacent to such sensitive parts.
The present invention addresses long-standing problems and concerns with attitude sensitivity in gas mass flow meters stemming from convective heat transfer outside a tube through which the gas is directed. It does so while also addressing the goals of high sensitivity and rapid responsiveness to changes in flow rate.